A transport refrigeration system used to control enclosed areas, such as the insulated box used on trucks, trailers, containers, or similar intermodal units, functions by absorbing heat from the enclosed area and releasing heat outside of the box into the environment. The transport refrigeration system commonly includes a compressor to pressurize refrigerant vapor, and a condenser to cool the pressurized vapor from the compressor, thereby changing the state of the refrigerant from a gas to a liquid. Ambient air may be blown across the refrigerant coils in the condenser to effect the heat exchange. The transport refrigeration system further includes an evaporator for drawing heat out of the box by drawing or pushing return air across refrigerant-containing coils within the evaporator. This step vaporizes any remaining liquid refrigerant flowing through the evaporator, which may then be drawn through a suction modulation valve (SMV) and back into the compressor to complete the circuit. The system may include a thermostatic expansion valve (TXV) in the refrigerant line upstream of the evaporator, which is responsive to the superheat generated in the evaporator (superheat being defined as the difference between the sensed vapor temperature and the saturation temperature at the same pressure). The transport refrigeration system also commonly includes an electric generator adapted to produce AC current at a selected voltage and frequency to operate a compressor drive motor driving the refrigeration compressor.
Some refrigeration systems, including transport refrigeration, require operation at reduced capacity to hold product within a very narrow temperature range. In some cases suction modulation is used to reduce and regulate capacity. This affects suction and discharge temperatures. When suction modulation occurs at high ambient temperatures, the refrigerant supplied to the compressor may be too hot, absent some correcting measures, resulting in compressor discharge temperatures that are too high.
Further, refrigeration systems that operate at low suction density and low mass flow conditions coupled with high compression ratios require additional compression temperature controls. In other refrigeration systems, such as mobile container systems used in tropical climates, a high ambient temperature adversely affects the temperature of the refrigerant, particularly the compressor discharge temperature. If discharge temperatures are not prevented from getting too hot, the compressor lubricant can break down and ultimately cause failure of the compressor.
Typical methods for controlling compressor discharge temperature include injecting liquid refrigerant by use of a liquid injection circuit via the economizer/vapor injection port on the compressor. One approach to injecting liquid refrigerant is by a solenoid-operated valve, commonly referred to as a quench valve. The quench valve bypasses the evaporator, that is, the liquid line tees off upstream of the evaporator and dumps in at the compressor suction inlet.
Unfortunately, refrigeration systems utilizing a quench valve have increased complexity, which increases cost. The increased complexity also makes system packaging more difficult in the confined space of a transport refrigeration system. Further, additional control parameters must be designed and implemented into the system controller.
Another drawback of systems utilizing a quench valve is that the liquid refrigerant bypasses the evaporator, thereby decreasing system efficiency. Also, the compressor superheat is more difficult to control with the use of a solenoid valve because large slugs of liquid are dumped into the suction inlet of the compressor. Too much liquid refrigerant can also result in floodback to the compressor and can ultimately cause failure of the compressor.